The invention concerns rotating joints used to transmit medium frequency or intermediate frequency signals between a rotating antenna and its support which is fixed, and especially rotating joints used in radar antennas.
Many radars use rotating antennas which are designed to first emit high-powered signals at very high frequencies and then to receive signals reflected by objects, these signals being very low-powered. Before these signals are processed in order to extract the information that they contain, they are first amplified to very high frequency and then transposed to a medium or intermediate frequency of about 60 megahertz at which they are again amplified. These different frequency amplification and transposing operations are performed on the antenna itself in a part of the radar called the "R.F. head". For the signals to be transmitted from the "R.F. head", which is on a rotating support, to the signal processing device itself, which is placed on a fixed support at a certain distance from the antenna, it is necessary to provide for a rotating joint which does this transmission.
Depending on the characteristic of the radar and, hence, of the radar antenna, the signals of several parallel reception channels should be transmitted simultaneously without the channels interfering with one another: this means that they should be insulated or decoupled by suitable shielding.
Broadly speaking, a prior art rotating joint has two cylindrical parts which are arranged concentrically and are hinged by means of ball bearings so that they rotate with respect to each other. These two parts are separated on a certain length by a ring-shaped space which is designed to accommodate several pairs of matched circuits, each corresponding to a reception channel. Each pair of matched circuits consists of two concentric, flat rings, the small ring being solidly joined to the internal cylindrical part and the big ring being solidly joined to the external cylindrical part. To make the coil of each matched circuit, a metallic strip is deposited on the same side of each ring, in following its internal and external rims, said strip being configured so as to form an open electrical winding, the ends of which are designed to receive appropriate external connections. Thus, one end is connected to the ground by means of the metallic strip placed on the internal rim of the small ring and on the external rim of the big ring, while the other end is connected to a coaxial conductor. By this mechanical and electrical arrangement, only the parts facing the metallic strip of each ring each constitute a coil of the matched circuit. The capacitors of the matched circuits result from the distributed capacitance of the windings and the addition of a capacitor with a defined capacitance.
The signal to be transmitted is applied to the ends of a matched circuit of one ring of a pair and is transmitted by a mutual inductance effect to the ends of the matched circuit of the other ring of the pair. Since the rings are concentric, this transmission takes place regardless of their respective angular positions.
To set up several channels, several pairs of rings are stacked, while being separated by shields also consisting of pairs of rings, the spacing between the rings of the pairs of matched circuits and the rings of the shields being obtained by spacers. These spacers are metallic and are in contact, firstly, with the concentric cylindrical parts of the rotating joint and, secondly, with the internal rim of the small ring and the external rim of the big ring. These different contacts constitute the electrical ground connection which is reinforced by screws that fix these spacers to the corresponding cylindrical parts of the rotating joint.
The rotating joint that has just been described has a number of drawbacks. Firstly, the pass-band of the channels is at more than 20 megahertz with attenuation of three decibels, and the insertion loss is about half a decibel. A limitation of this kind is incompatible with the characteristics required for modern radars.
Furthermore, the stacking of a large number of spacers requires high precision in their making and assembly because it is important for the rings of a pair not to have their position shifted because this would result in lower mutual inductance. This problem is all the more difficult to resolve as the number of channels is great, as is the case with modern radar installations where the rotating joint is often designed for twelve channels in parallel. Furthermore, the thickness of the spacers is such that it leads to the rotating joint having dimensions close to one meter.
Again, the ground link is poorly established because the metallic ribbon is deposited on only one side of the ring and is, therefore, in contact only with the spacers on the same side and these spacers are, moreover, connected to the ground of the rotating joint by fastening screws.